A photo of the former adversaries shows U.S. Secretary of State John Kerry shaking hands with Cuban Foreign Minister Bruno Rodriguez, slight smiles playing across their faces.

The image of this highest-level encounter between governments of the United States and communist Cuba in decades – taken late Thursday on the eve of a regional summit and tweeted by the State Department – captures a moment in the U.S. charm offensive to speed normalized relations with the island nation and to placate critics of the deal.

Kerry and his boss, U.S. President Barack Obama, are joining other international leaders for the two-day Summit of the Americas, which opens Friday.

A senior State Department official said Kerry's historic talk with Rodriguez was "very constructive," adding both sides "agreed they made progress."

The official did not specify any advances, but stressed that both sides would "continue to work to resolve outstanding issues."

It’s possible Obama, who arrived late Thursday, could meet Cuban President Raul Castro on the sidelines of the gathering, according to administration officials.